Correct CommitBuilder, TagBuilder method to be build()

The correct names for these is build(), as that is what a Java
developer will expect given the "builder" pattern.

Bug: 323541
Change-Id: I35042bdc95a955beeaee29e54bde10e4240b2a71
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Reviewed-by: Chris Aniszczyk <caniszczyk@gmail.com>